Evolution of glucuronoxylan side chain variability in vascular plants and the counter-adaptation of pathogenic cell-wall-degrading hydrolases

Abstract

SummaryPlant cell wall polysaccharides such as xylan not only contribute to cell shape, but also hinder pathogenic and biotechnological attempts to saccharify the wall. This recalcitrance derives substantially from polysaccharide structural complexity, driven by biosynthetic glycosyltransferases. Glucuronic acid sidechains along the xylan backbone can be further decorated with arabinopyranose or galactose in certain tissues and species. We sought to identify the responsible glycosyltransferases.We identified the relevant locus from candidate Arabidopsis mutants by polysaccharide profiling and reverse genetics. We demonstrated different activities of two paralogues in eucalyptus by heterologous expression and investigated this neofunctionalisation by molecular phylogeny. Finally, we measured the activities of various GH30-familyendo-glucuronoxylanases on modified xylansin vitro.AtXAPT1 (AT1G68470) andEgXAPT (Eucgr.D00738) encodeXylanArabinoPyranosylTransferases, whereasEgXLPT (Eucgr.H00343) encodes aXylanGalactoPyranosylTransferase. While arabinopyranosyl decorations in native eucalyptus appear to be enriched in primary walls, overexpression of the eucalyptus enzymes in Arabidopsis can accomplish substantial changes in secondary wall xylan structure. Only some GH30 glucuronoxylanases have evolved to accommodate substituted glucuronic acid, perhaps through a single active site substitution.Our results demonstrate the adaptability of biosynthetic and degradative carbohydrate-active enzyme activities, providing insight into a plant-pathogen arms race and facilitating wall biotechnological utilisation.